1. Field of the Invention
The invention relates to the suppression of lead (Pb) dust generated during the manufacture of lead-acid batteries by coating pasted plates with a latex dispersion comprising ammonium sulfate (NH.sub.4).sub.2 SO.sub.4, copolymer and water. The invention also contemplates the composition of the latex dispersion and the coated plates.
2. Prior Art
The manufacture of lead acid batteries involves numerous steps which generate lead dust. In manufacturing battery plates, a metallic grid is pasted by pressing lead oxide mixed with water and dilute sulfuric acid into the voids of the grids. The process is commonly referred to as pasting the battery plate. Thereafter, the pasted grid is flash dried, cured, formed (initially charged), washed and dried. The resultant grid or battery plate is assembled to form a battery. The danger of lead emission from the pasted grid is very high when the pasted grid is subjected to the flash drying process and then transported to subsequent work stations. During the drying process, moisture rises to the plate surface and vaporizes. This results in a thin film of dust on the surface of the pasted grid which is easily lost to the environment. The step usually considered to produce the largest amount of fine lead dust pollution resides between the flash drying of the battery plate and the strap burning procedure.
It is well known that lead compounds accumulate in human bodies and become toxic at a level at approximately 0.5 mg. of lead per day. Accordingly, the battery industry has made numerous efforts to improve the air quality standard for the well being of battery industry employees. Further many governmental and health agencies establish air quality standards and regulations for industries utilizing lead compounds. Heretofore, the battery industry has failed to provides economical method of reducing lead dust in a battery factory without substantially reducing the performance of the battery.
U.S. Pat. No. 3,905,829 issued to Weissman et al utilizes a water soluble polymeric coating which, when applied to a pasted battery grid plate, lowers the emission of lead dust. The water soluble polymers embraced are, for example, polyvinyl alcohol, polyethylene oxide, alkyl cellulose, acrylic resins and water soluble starches. The coating is less than one micron in thickness so that the coating adequately provides for the transmission of oxygen and water vapor for the curing process. This coating is only a temporary coating on an unformed plate since the coating is substantially washed out during the formation of the plate. The failure to affect battery performance was considered unexpected since the use of polymeric materials on a lead acid battery plate was expected to increase electrical resistance, thereby necessitating a larger battery to attain the same electrical output as an uncoated battery plate.
U.S. Pat. No. 4,197,368 issued to Davis et al discloses adding sodium silicate to the lead paste mix before the grids are pasted to reduce lead dust created by the handling of the plates.
U.S. Pat. No. 4,232,100 reduces lead dust by coating the plates with a water insoluble plastics material (styrene and acrylic acid ester) delivered in the form of a diluted 50 percent aqueous disperson. The thickness of the coating is limited since the electrical properties of the plate would be degraded by a thick coating.
The electrical permeability of the coating is important as recognized by prior art disclosures whose purpose is to protect the plates against the effects of charging and discharging, viz. shedding and treeing.
U.S. Pat. No. 756,176 discloses a protective sheath consisting of castor oil, essence of turpentine and nitrates of cellulose. The sheath is made porous either by chemical or mechanical means.
U.S. Pat. No. 2,673,230 relates to a protective porous sheath formed by depositing particles, e.g. polystyrene, polyethylene, and causing the particles to unite on heating of the battery plate. The disclosure cautions not to excessively heat the plate so that the particles become molten in order to retain the small spaces between the particles.
U.S. Pat. No. 2,810,775 utilizes a protective web impregnated with a solution of a thermosetting resin. The resultant web is permeable to the electrolyte of the storage battery.
Canadian Pat. No. 664,290 teaches a latex coating formed by copolymerizing vinylidene chloride with a minor amount of an alkyl acrylate. The resultant film is sufficiently porous to be permeable to the electrolyte and to permit the passage of gases formed in reactions at the battery plates. The purpose of the film is to prevent excess shedding of the plate paste during battery cycling.
As is evident, the prior art has failed to establish an economical yet highly effective method of lowering lead dust in the workplace while also retaining electrical performance.
Others in the prior art have used a solution of ammonium sulfate to plug or seal lead compounds within a slitted vulcanized rubber tube. This was necessary because the lead components within the rubber tube were very fine and would leak out of the slit. This would result in the loss of a valuable component which had to be removed from the workplace and would result in a nonuniform product.
Therefore, it is an object of this invention to provide a system which overcomes the aforementioned inadequacies of the prior art and provides an improvement which is a significant contribution to the advancement of the pertinent art.
Another object of the present invention is to provide an improved process of reducing lead dust emanating from pasted plates during lead acid battery manufacturing procedures.
Another object of the present invention is to produce an improved coated lead acid battery plate with a low dusting characteristic and without substantially altering electrical performance.
Another object of the present invention is to provide an improved coating composition suitable for coating pasted lead-acid battery plates.
The foregoing has outlined some of the more pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the detailed description describing the preferred embodiment in addition to the scope of the invention defined by the claims.